The invention is directed to a yellow gold alloy for firing on porcelain for dental purposes according to German OS 24 24 575.8 and related Knosp U.S. Pat. No. 4,062,676 which contains beside gold, platinum, indium, tin and rhodium and/or tantalum and/or tungsten, but which also contains palladium. The entire disclosure of the Knosp U.S. patent is hereby incorporated by reference and relied upon.
In the dental prosthesis it is customary to cover crowns and bridges of special noble metal alloys with porcelain. Standing in the foreground is the idea of combining the good mechanical properties of the alloy and the aesthetic appearance and the tissue compatability of porcelain.
In the infancy of this art, it was tried to fire ceramic compositions or caps and structures of platinum-iridium alloys. Because of the poor workability of these high melting alloys and the incomplete harmonization of the properties with those of porcelain, however, there very frequently occur failures.
In the last years a series of special alloys have become known which, based on their physical and mechanical properties, substantially satisfy the requirements of the firing art. There are used alloys which contain 70 to 90% gold, 5 to 15% platinum, 0.5 to 10% palladium, 0.1 to 2% indium, 0.1 to 2% tin, as well as small additions of silver, copper, iron, iridium and rhenium.
All of the alloys, however, have the disadvantage that they exhibit a gray color. However, in dental alloys a yellow gold color is very much desired. Especially in visible, non-faced parts of a metal structure finished by a fired alloy a yellow color is desired for aesthetic reasons. Also, at those places of the structure on which the ceramic is molded somewhat thinly a living appearance is caused by a yellow color. Therefore, in recent years, there has not been a lack of attempts to produce alloys for firing which are considerably yellower in color than those previously known and which simultaneously conform in their physical and mechanical properties to the necessary industrial requirements.
There are known alloys which besides the above-mentioned constituents also contain nickel, titanium and zinc, as well as large amounts of copper. Besides, it was tried to attain a deepening of the yellow gold color by increasing the gold content and lowering the palladium content. However, all of these procedures can merely produce alloys which only come a little nearer the yellow gold color than the previous customary firing alloys. It has further been found that the properties of the alloys are unfavorably influenced through the named additives. For example, titanium causes a slag formation in melting and casting because of the high negative enthalpy of formation of its oxide; copper and, particularly zinc, very strongly lower the solidus temperature so that there can no longer be guaranteed a sufficient heat resistance of the alloy during the firing of porcelain. Furthermore, because of the ready volatility of zinc oxide, zinc has an unfavorable effect on the adhesion between alloy and porcelain.
It is known that palladium above all others very disadvantageously influences the color of gold alloys. Alloys containing over 70% of gold which contain palladium but no, or only a very little, copper and zinc appear gray or at most show a pale yellow color. In firing alloys palladium has the main task of increasing the solidus temperature and in combination with indium and/or tin makes possible a tempering since it is likewise known that in gold alloys containing additions of indium and/or tin the hardness depends very greatly on the palladium content. It was previously believed that the addition of palladium could not be eliminated.
In German OS 24 24 575.8 and Knosp U.S. Pat. No. 4,062,676, however, it has been proposed to use palladium free gold alloys for firing porcelain for dental purposes in order to obtain an intensive gold color. To obtain the most favorable mechanical properties, these alloys contain rhodium and/or tantalum and/or tungsten. The alloys according to German OS 24 24 575.8 contain 80 to 90% gold, 5 to 15% platinum, 0.1 to 2% indium, 0 to 2% tin, 0.05 to 0.5% iridium as well as 0.5 to 3% rhodium and/or 0.1 to 2% tantalum and/or 0.1 to 2% tungsten. The alloys according to Knosp U.S. Pat. No. 4,062,676 contain 60 to 90% gold, 5 to 35% platinum, 0.1 to 3% indium, 0 to 10% palladium, 0.5 to 3% rhodium, 0 to 3% tin, 0.1 to 2% tantalium and/or tungsten and 0.3 to 2% zinc.
Of course these alloys have a sufficiently high solidus temperature for the normal case, but for several use situations this is still too low.
Therefore, it was the problem of the present invention to find gold alloys for firing on porcelain for dental purposes which beside having an intensive gold color have higher solidus temperatures than the alloys according to German OS 24 24 575.8 and Knosp U.S. Pat. No. 4,062,676.